In the absence of preionization, gas discharge pulsed lasers such as ArF, XeF and KrF excimer lasers and molecular fluorine lasers are generally unstable. In particular, without preionization, the glow discharge that is required for suitable laser emission does not occur. On the other hand, preionization which results in a uniform distribution of electrons in the gas discharge region immediately before the main gas discharge occurs can be used to produce high quality, consistent laser pulses.
Early attempts at preionization included the use of spark preionization which created a high current, localized, intense spark which had the undesirable effect of vaporizing electrode material and consuming and contaminating the active laser gas. More recently, non-sparking corona discharge preionization has been developed and employed to create a substantially uniform emission of radiation which typically includes UV and X-ray wavelengths. Corona discharge producing a uniform preionization can result in a good beam profile without hot spots, high energy stability, improved electrode lifetimes and reduced halogen consumption. In one such method, a corona discharge is produced by establishing a potential difference across a dielectric material.
In more geometrical terms, a typical gas discharge laser may employ a pair of spaced apart, elongated (e.g. 60 cm long) main discharge electrodes to initiate lasing in a gaseous material. A blower may be provided to circulate a laserable gaseous medium past the electrodes to quickly exhaust gas from the discharge region after a pulse and present a fresh portion of gas to the electrodes for the next pulse. Thus, a rather large elongated, somewhat rectangular discharge volume of fresh gaseous media is required to be uniformly preionized, immediately prior to a discharge between the main electrodes.
For the above described geometry, an effective way to create a substantially uniform preionization generally involves the use of an elongated tube made of dielectric material that is aligned parallel to the discharge electrodes and positioned near the discharge region. A conductive preionization electrode (typically made of copper or brass), can then be placed in the bore of the tube and used to create a potential difference between the preionization electrode and one of the main discharge electrodes. This potential difference extends across the dielectric tube radially and results in a substantially uniform emission of photons that are emitted from the outer surface of the tube.
In one arrangement, an aluminum chamber housing is employed that envelops the gaseous media and the discharge region. Each end of the preionization electrode is then mechanically, and in some cases, electrically connected to the housing, which is then grounded. Thus, a potential difference between the preionization electrode and a main electrode is established when the main electrode is biased relative to ground.
An important factor that must be considered when designing a preionization system is the response of the various preionization structures to heat generated in the laser chamber. With the above described structure, one primary failure mode has been identified as the cracking of the relatively brittle dielectric tube due to thermal expansion of the brass/copper preionization electrode. In particular, both of these materials have a relatively high linear thermal expansion coefficient (LTEC) (i.e. the fractional change in length of a bar per degree of temperature change). For example, Brass has an LTEC, of about 19×10−6 per degree Celsius and Copper has a LTEC of about 17×10−6 per degree Celsius, as compared to aluminum which has an LTEC, of about 23×10−6 per degree Celsius. In operation, the preionization electrode is typically heated to a higher temperature than the aluminum housing, and as a consequence, the preionization electrode may expand much more than the stainless steel housing upon exposure to heat, and this expansion differential may cause the electrode to bow and crack the dielectric tube.
With the above considerations in mind, Applicants disclose preionization systems and methods for providing a substantially uniform preionization volume while accommodating the heat generated in the laser chamber.